Who was the longest reigning male British monarch?

George III (George William Frederick; 4 June 1738 – 29 January 1820) was King of Great Britain and Ireland from 25 October 1760 until the union of the two countries on 1 January 1801, after which he was King of the United Kingdom of Great Britain and Ireland until his death.

In the later part of his life, George III had recurrent, and eventually permanent, mental illness. Although it has since been suggested that he had the blood disease porphyria, the cause of his illness remains unknown. After a final relapse in 1810, a regency was established, and George III's eldest son, George, Prince of Wales, ruled as Prince Regent. On George III's death, the Prince Regent succeeded his father as George IV.

George III lived for 81 years and 239 days and reigned for 59 years and 96 days: both his life and his reign were longer than those of any of his predecessors. Only Victoria and Elizabeth II have since lived and reigned longer.